1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and more particularly, to a belt fixing mechanism that uses a belt as a fixing member.
2. Description of the Related Art
As is well known, an electrophotographic image forming apparatus outputs a copy image by the following processes.
That is, a latent image formed on a photosensitive element that is a latent image carrier is processed to be visualized by toner, and the toner image is transferred onto a recording medium such as a recording sheet and then fixed to the recording medium, whereby the copy image is output.
A fixing method used in an image forming apparatus includes a heat roller fixing method, a belt fixing method, a film fixing method, and an electromagnetic induction heating fixing method.
In the heat roller fixing method, used are a fixing roller and a pressing roller that face each other across a conveying path of a recording sheet and abut each other. In this method, a toner image melts and permeates the recording sheet by the action of heat from a heat source provided inside the fixing roller and a pressure corresponding to the pressure applied from the pressing roller. The phenomenon that the toner image melts and permeates the recording sheet is the same in the fixing methods using the following configurations.
In the belt fixing method, used are a fixing belt that serves as a good heat conductor in place of a fixing roller, a pressing roller, a roller on which the belt is wound, and a heat source for the belt (for example, Japanese Patent Application Laid-open No. 2004-286922).
In the film fixing method, used are a fixing belt that serves as a good heat conductor in place of a fixing roller, a pressing roller, a roller on which the belt is wound, and a heat source for the belt (for example, Japanese Patent Application Laid-open No. 2010-079309).
In the electromagnetic induction heating fixing method, used is a configuration in which a heating member is provided with an electromagnetic induction coil that improves heat generation efficiency (for example, Japanese Patent Application Laid-open No. 2004-286922).
A fixing method has the following requirements.
The requirements include to shorten a warm-up time (a time for reaching a given temperature (reload temperature) making it possible to perform a printing process), from a state of normal temperature when powered on, for example, and to shorten a first print time (a time for performing print preparation, performing printing operation and finishing discharging a sheet after a print request is received).
In a fixing device, fixing failure may occur due to the following reasons.
An image forming apparatus is capable of high-speed processing. When the number of fixing sheets per unit time, i.e., the number of sheets passing through the fixing device per unit time is increased by high-speed processing, quantity of heat supplied to a fast-moving recording sheet also needs to be increased. This is to provide the recording sheet with quantity of heat necessary for fixing conforming to the shortened time during which the recording sheet passes through the fixing device.
However, when the necessary quantity of heat is not secured at the start of continuous printing, a large drop in temperature occurs, and when a sheet passes through while the necessary quantity of heat is not reached in speed-up continuous printing, fixing failure may result.
Furthermore, due to the speeding-up of image forming apparatus, the number of sheets to pass through per unit time is increased and the necessary quantity of heat is increased. This may lead to what is known as temperature drop in which the quantity of heat is insufficient particularly at the start of continuous printing, and may lead to fixing failure when sped up.
Meanwhile, separate from the above-described fixing methods, known is a fixing method referred to as a SURF fixing method in which a ceramic heater is used.
In the SURF fixing method, used is a configuration in which only a nip portion is locally heated and other portion is not heated. In the SURF fixing method, compared with a belt-type fixing device, lowering of heat capacity and downsizing is possible, whereby a time for rising to a given temperature and the first print time can be shortened but there are following problems.
That is, in the SURF fixing method, because the portion other than the local portion are not heated, the belt is in a coldest state at the entrance of a nip for a sheet or the like, whereby fixing failure is likely to occur. Particularly, a high-speed device has the problem that fixing failure is more likely to occur because the rotation of the belt is fast and thus the heat dissipation of the belt at the portion other than the nip portion becomes large.
Accordingly, to deal with such a problem, proposed is a fixing device that can achieve good fixing performance, in a configuration using an endless belt, even when the device is mounted on a high-producing image forming apparatus (for example, Japanese Patent Application Laid-open No. 2007-334205).
The fixing device disclosed in Japanese Patent Application Laid-open No. 2007-334205 uses a configuration illustrated in FIG. 9.
The fixing device includes an endless belt 100, a pipe-shaped metal heat conductor 200 arranged inside the endless belt 100, a heat source 300 arranged inside the metal heat conductor 200, and a pressing roller 400 that abuts the metal heat conductor 200 via the endless belt 100 to form a nip portion N by which a recording sheet can be clamped and conveyed.
The endless belt 100 is rotated by the rotation of the pressing roller 400, and at this time, the metal heat conductor 200 guides the movement of the endless belt 100. Furthermore, heating the endless belt 100 by the heat source 300 inside the metal heat conductor 200 via the metal heat conductor 200 enables the whole endless belt 100 to be heated. Consequently, the first print time from a heating standby state can be shortened, and the insufficiency of quantity of heat in a high-speed rotation state can be eliminated.
However, to further improve energy saving and the first print time, it is necessary to further improve heat efficiency. Accordingly, proposed is a configuration in which an endless belt is not indirectly heated via a metal heat conductor (the member indicated by the reference numeral 200 in FIG. 9), but is directly heated (without the intervention of the metal heat conductor) (for example, Japanese Patent Application Laid-open No. 2007-233011).
FIG. 10 illustrates the configuration of a fixing device disclosed in Japanese Patent Application Laid-open No. 2007-233011.
In FIG. 10, in the fixing device, the above-described pipe-shaped metal heat conductor is removed from the inside of the endless belt 100, and in place of that, a plate-shaped nip forming member 500 is positioned to face the pressing roller 400.
This configuration allows heating up the endless belt 100 directly by the heat source 300 at the portion other than where the nip forming member 500 is arranged, thereby substantially improving the efficiency of heat transfer and thus reducing the power consumption.
As a consequence, the first print time from the heating standby state can be further shortened. Furthermore, cost reduction can be expected due to the metal heat conductor not being provided. In the fixing device, the nip forming member 500 is supported by a supporting member 600 of stainless-steel or the like to improve the strength of the nip forming member 500 against the applied pressure by the pressing roller 400.
However, in the fixing device illustrated in FIG. 10, for the purpose of reducing a heat dissipation area to further improve the heat efficiency, a belt of a small diameter of about 30 millimeters is used as the endless belt 100.
In such a configuration, the size of the supporting member 600 arranged inside the endless belt 100 tends to be small. As a result, if a sufficient strength of the nip forming member 500 cannot be achieved and the deflection of the nip forming member 500 is caused by the applied pressure of the pressing roller 400, unevenness occurs in the distribution of contact pressure or the width of the nip in the nip portion N, whereby fixing failure may occur.
In view of the situations of the above-described conventional fixing devices, there is a need to provide a fixing device configured to improve the strength of a supporting member that supports a nip forming member and to prevent the deflection of the nip forming member, and an image forming apparatus provided with the fixing device.